export  function  imgPreview (file, callBack) {

	let maxsize = 500 * 1024;

    // let self = that  

    if (!file) return

    if (!/\/(?:jpeg|png|gif)/i.test(file.type)) return

    let reader = new FileReader()

    reader.onload = function() {
        let result = this.result
        let img = new Image(); 
        img.src = result
        
        // 如果图片大小小于500kb，则直接上传
        if (result.length <= maxsize) {
            callBack(file)

            img = null
			return
        }

        // 图片加载完毕之后进行压缩，然后上传
        if (img.complete) {
            defaultCallback();
        } else {
            img.onload = defaultCallback;
        }

        function defaultCallback() {
            let base64Str = _method.compressImg(img)
            let compressImg = _method.base64ImgToFile(base64Str, file.type, file.name);
            callBack(compressImg);

            img = null;
        }

    }
    
    reader.readAsDataURL(file);

}

// ---------------- 以下是私有方法, 内部调用 -------------
let _method = {
    // 压缩图片
    compressImg(img) {
        
        let canvas = document.createElement("canvas");  
        let ctx = canvas.getContext('2d');  
             //瓦片canvas  
        let tCanvas = document.createElement("canvas");  
        let tctx = tCanvas.getContext("2d");  

        let initSize = img.src.length
        let width = img.width
        let height = img.height

        // 如果图片大于四百万像素，计算压缩比并将大小压至400万以下
        let ratio;

        if ((ratio = width * height / 4000000) > 1) {
            ratio = Math.sqrt(ratio);
            width /= ratio;
            height /= ratio;
        } else {
            ratio = 1;
        }

        canvas.width = width;
        canvas.height = height;
        

        // 铺底色
        ctx.fillStyle = "#fff";
        ctx.fillRect(0, 0, canvas.width, canvas.height);

        // 如果图片像素大于100万则使用瓦片绘制
        let count;
        if ((count = width * height / 1000000) > 1) {
            count = ~~(Math.sqrt(count) + 1); // 计算要分成多少块瓦片

            // 计算每块瓦片的宽和高
            let nw = ~~(width / count)
            let nh = ~~(height / count)

            tCanvas.width = nw
            tCanvas.height = nh

            for (let i = 0; i < count; i++) {
                for (let j = 0; j < count; j++) {
                    tctx.drawImage(img, i * nw * ratio, j * nh * ratio, nw * ratio,
                        nh * ratio, 0, 0, nw, nh);

                        ctx.drawImage(tCanvas, i * nw, j * nh, nw, nh);
                }
                
            }
        } else {
            ctx.drawImage(img, 0, 0, width, height);
        }

        // 进行最小压缩
        let ndata = canvas.toDataURL('image/jpeg', 0.5);
        
        tCanvas.width = tCanvas.height = canvas.width = canvas.height = 0;

        return ndata;
    },
    // 将base64的图片转成二进制对象
    base64ImgToFile(basestr, type, fileName) {
        
       


        let text = window.atob(basestr.split(",")[1]);
        let buffer = new ArrayBuffer(text.length);
        let ubuffer = new Uint8Array(buffer);
        let pecent = 0, loop = null;
        let array = [];


        for (let i = 0; i < text.length; i++) {
           
            array.push(text.charCodeAt(i));
        }
    
        let blob = new Blob([new Uint8Array(array)], {type: 'image/jpeg'})
        return new File([blob], fileName)
    }
}
